The location and nature of extracellular barriers to colloidal substances at a variety of locations in the nervous system was determined. At the meningeal covering of the brain, at the special ependyma covering the median eminence and area postrema, and at the reticular lamina in the organ of Corti, extensive systems of tight junctions form continuous bands surrounding cells and occluding extracellular spaces. It is these bands of junctions which control the movement of colloidal and probably ionic substances in intervening extracellular spaces. No tight junctions are found at the astrocytic border of the brain but other membrane specializations revealed by the freeze-fracture technique suggest that these cells also participate in the blood-brain barrier system. These data on the cellular basis of brain barriers, as well as the new structural approaches used in these studies, afford a basis to design experimental studies of a variety of clinically important conditions which depend on or produce damage to brain barriers. They also give rise to new ideas about the roles of various cells associated with neurons and about the function of barrier systems in special regions, such as the organ of Corti.